Supporting Distinct Single-Input Single-Output (siso) Services in a Multiple-Input Multiple-Output (mimo) Baseband Circuit, Particularly Suited for a Distributed Antenna System (das)

1. A multiple-input multiple-output (MIMO) baseband circuit configured to support distinct single-input single-output (SISO) services, comprising: a first MIMO path configured to distribute a first SISO signal; a second MIMO path configured to distribute a second SISO signal; a first quadrature modulator comprising a first SISO signal input coupled to the first MIMO path, the first SISO signal input configured to receive the first SISO signal; and a second quadrature modulator comprising a second SISO signal input coupled to the second MIMO path, the second SISO signal input configured to receive the second SISO signal; wherein: the first quadrature modulator is configured to modulate the first SISO signal to a first radio frequency (RF) band based on a modulation frequency; the second quadrature modulator is configured to modulate the second SISO signal to a second RF band that is different from the first RF band based on the modulation frequency; the first RF band comprises a first center frequency and a first bandwidth; the second RF band comprises a second center frequency and a second bandwidth; and the modulation frequency resides between the first center frequency of the first RF band and the second center frequency of the second RF band.

2. The MIMO baseband circuit of claim 1 , wherein the modulation frequency resides in the middle of the first center frequency of the first RF band and the second center frequency of the second RF band.

3. The MIMO baseband circuit of claim 1 , wherein: the first SISO signal is received by the first SISO signal input on a first intermediate frequency (IF) band; the second SISO signal are received by the second SISO signal input on a second IF band; the first IF band comprises a first IF center frequency and a first IF bandwidth, wherein: the first IF center frequency is determined by subtracting the modulation frequency from the first center frequency; and the first IF bandwidth is equal to the first bandwidth of the first RF band; and the second IF band comprises a second IF center frequency and a second IF bandwidth, wherein: the second IF center frequency is determined by subtracting the modulation frequency from the second center frequency; and the second IF bandwidth is equal to the second bandwidth of the second RF band.

4. The MIMO baseband circuit of claim 1 , wherein the modulation frequency is lower than the first center frequency of the first RF band.

5. The MIMO baseband circuit of claim 1 , wherein the modulation frequency is higher than the second center frequency of the second RF band.

6. The MIMO baseband circuit of claim 1 , wherein: the first SISO signal comprises a first in-phase (I) signal and a first quadrature (Q) signal; and the second SISO signal comprises a second I signal and a second Q signal.

7. The MIMO baseband circuit of claim 6 , wherein: the first quadrature modulator is further configured to modulate the first I signal and the first Q signal to generate a first RF signal for communicating on the first RF band; and the second quadrature modulator is further configured to modulate the second I signal and the second Q signal to generate a second RF signal for communicating on the second RF band.

8. A method for support distinct single-input signal-output (SISO) services in a multiple-input multiple-output (MIMO) baseband circuit, comprising: receiving a first SISO signal on a first MIMO path; receiving a second SISO signal on a second MIMO path; modulating the first SISO signal to a first radio frequency (RF) band based on a modulation frequency; modulating the second SISO signal to a second RF band that is different from the first RF band based on the modulation frequency; determining a first center frequency and a first bandwidth of the first RF band; determining a second center frequency and a second bandwidth of the second RF band; and determining the modulation frequency based on the first center frequency and the second center frequency, wherein the modulation frequency resides between the first center frequency and the second center frequency.

9. The method of claim 8 , further comprising determining the modulation frequency by averaging the first center frequency and the second center frequency.

10. The method of claim 9 , further comprising: receiving the first SISO signal on a first intermediate frequency (IF) band, wherein the first IF band comprises a first IF center frequency and a first IF bandwidth; receiving the second SISO signal on a second IF band, wherein the second IF band comprises a second IF center frequency and a second IF bandwidth.

11. The method of claim 10 , further comprising: determining the first IF center frequency by subtracting the modulation frequency from the first center frequency; determining the first IF bandwidth as being equal to the first bandwidth; determining the second IF center frequency by subtracting the modulation frequency from the second center frequency; and determining the second IF bandwidth as being equal to the second bandwidth.

12. A distributed antenna system (DAS), comprising: a head-end equipment (HEE) configured to receive a first single-input single-output (SISO) signal and a second SISO signal; and a plurality of remote antenna units (RAUs) communicatively coupled to the HEE by an optical fiber-based communications medium; wherein at least one RAU among the plurality of RAUs comprises an adaptive multiple-input multiple-output (MIMO) communication system configured to: receive and modulate the first SISO signal to generate a first radio frequency (RF) signal for distribution on a first RF band; and receive and modulate the second SISO signal to generate a second RF signal for distribution on a second RF band that is different from the first RF band, wherein the adaptive MIMO communication system comprises: a MIMO baseband circuit, comprising: a first MIMO path configured to distribute the first SISO signal; a second MIMO path configured to distribute the second SISO signal; a first quadrature modulator comprising a first SISO signal input coupled to the first MIMO path, the first SISO signal input configured to receive the first SISO signal; and a second quadrature modulator comprising a second SISO signal input coupled to the second MIMO path, the second SISO signal input configured to receive the second SISO signal; wherein: the first quadrature modulator is configured to modulate the first SISO signal to generate the first RF signal for distribution on the first RF band based on a modulation frequency; and the second quadrature modulator is configured to modulate the second SISO signal to generate the second RF signal for distribution on the second RF band that is different from the first RF band based on the modulation frequency; a first in-phase (I)/quadrature (Q) signal generator (I/Q signal generator) configured to provide the first SISO signal to the first MIMO path; a second I/Q signal generator configured to provide the second SISO signal to the second MIMO path; a first filter coupled to the first quadrature modulator, the first filter configured to pass the first RF signal in the first RF band; and a second filter coupled to the second quadrature modulator, the second filter configured to pass the second RF signal in the second RF band.

13. The DAS of claim 12 , wherein: the first RF band comprises a first center frequency and a first bandwidth; and the second RF band comprises a second center frequency and a second bandwidth.

14. The DAS of claim 13 , wherein the modulation frequency resides between the first center frequency of the first RF band and the second center frequency of the second RF band.

15. The DAS of claim 14 , wherein the modulation frequency resides in the middle of the first center frequency of the first RF band and the second center frequency of the second RF band.

16. The DAS Hof claim 15 , wherein: the first SISO signal is received by the first SISO signal input on a first IF band; the second SISO signal is received by the second SISO signal input on a second IF band; the first IF band comprises a first IF center frequency and a first IF bandwidth, wherein: the first IF center frequency is determined by subtracting the modulation frequency from the first center frequency; and the first IF bandwidth is equal to the first bandwidth of the first RF band; and the second IF band comprises a second IF center frequency and a second IF bandwidth, wherein: the second IF center frequency is determined by subtracting the modulation frequency from the second center frequency; and the second IF bandwidth is equal to the second bandwidth of the second RF band.

17. The DAS of claim 16 , wherein the modulation frequency is lower than the first center frequency of the first RF band and the modulation frequency is higher than the second center frequency of the second RF band.

18. The DAS of claim 13 , wherein: the first SISO signal comprises a first I signal and a first Q signal; the second SISO signal comprises a second I signal and a second Q signal; the first quadrature modulator is further configured to modulate the first I signal and the first Q signal to generate the first RF signal for communicating on the first RF band; and the second quadrature modulator is further configured to modulate the second I signal and the second Q signal to generate the second RF signal for communicating on the second RF band.

19. The DAS of claim 18 , wherein: a first IF modulator is configured to: generate the first I signal based on a first digital I signal received from a digital signal processor (DSP); and generate the first Q signal based on a first digital Q signal received from the DSP; and a second IF modulator is configured to: generate the second I signal based on a second digital I signal received from the DSP; and generate the second Q signal based on a second digital Q signal received from the DSP.

20. The DAS of claim 12 , wherein: a first IF modulator comprises a zero-IF modulator configured to provide the first SISO signal to the first MIMO path; the first IF modulator comprises a complex-IF modulator configured to provide the first SISO signal to the first MIMO path; a second IF modulator comprises a zero-IF modulator configured to provide the second SISO signal to the second MIMO path; and the second IF modulator comprises a complex-IF modulator configured to provide the second SISO signal to the second MIMO path.

21. The DAS of claim 14 , wherein: the HEE comprises an electrical-to-optical (E/O) converter and an optical-to-electrical (O/E) converter; and each of the plurality of RAUs comprises a respective O/E converter and a respective E/O converter.